Field of the Invention
The present invention relates to a spark plug.
Description of Related Art
Spark plugs have thus far been designed for internal combustion engines of apparatuses such as an automobile, a cogeneration system, and a gas transfer pump. Such spark plugs include a central electrode and a ground electrode, between which a spark discharge gap is interposed. The air-fuel mixture is ignited by a spark discharge in the spark discharge gap.
Development of highly efficient engines or maintenance-free engines requires life extension of such spark plugs, so that the spark plugs include a tip made of a noble metal such as an iridium (Ir) alloy at an opposing portion or a spark discharge portion of the central electrode, which faces the spark discharge gap.
Here, a noble metal tip (such as Ir alloy) and a central-electrode base material (such as Ni alloy) have a large difference in coefficient of thermal expansion. To prevent the tip from being separated due to thermal stress, a fusion layer having a coefficient of thermal expansion that is substantially in the middle between the coefficients of thermal expansion of the noble metal tip and the central electrode base material is formed by laser welding. The thermal stress is thus reduced to tightly connect the noble metal tip and the central electrode base material together. In addition, a known spark plug includes a fusion layer, whose dimensional relationships between, for example, a width and a tip height, are adjusted so that the spark plug includes a durable firing end while the noble metal tip and the central electrode base material are sufficiently tightly connected together (for example, PTL 1).